Single most precise measurement of the top quark mass

The figure shows the agreement between the data and a fit using the “signal + background” template.

Like competitors in the Olympics, we at CDF try to do our best for each measurement we make. Thus we now have a new measurement of the top quark mass that has replaced our old one as the single most precise measurement of the top quark mass. Earlier this year both the CDF and DZero collaborations measured the mass of the W boson to very high precision. The combination of the top quark and W measurements provides an indirect determination of the Higgs particle’s mass. This indirect determination is consistent with the recently discovered Higgs-like particle at CERN and has been slightly improved by our new measurement.

When top quarks are created in Tevatron collisions, they almost immediately decay into a bottom quark and a W boson and are most often produced in pairs. The top quark is best measured when one W decays into a neutrino and a charged lepton (electron or muon) and the other top quark decays into a pair of jets (light quarks). This so-called lepton-plus-jets decay mode contains four jets of which two are bottom quark jets. Our reconstruction of the top mass relies heavily on our ability to measure jet energy. Scientists fine-tuned the energy measurement using an advanced mathematical algorithm specifically formulated to correct jet energies of the top pair events.

CDF physicists used the full Run II data set for this measurement. For each event, they reconstructed the top quark mass two different ways and reconstructed the W boson mass from a pair of jets. They used this W boson mass distribution as an additional calibration for the energy of jets in the CDF detector, thereby reducing the single largest source of systematic uncertainty.

Then they compared these three distributions, reconstructed from the data, to simulated samples. The simulated samples require that we understand not only the process we are measuring but also the backgrounds. The most important backgrounds are the production of W’s + jets and multijets. Scientists produced simulated samples with different known values of the top quark mass. Finding the best match between the simulated samples and the data allows us to determine the top quark mass. This procedure is called the template method.

Using these strategies, we have made the world’s most precise measurement of the top quark mass, 172.85 ± 1.11 GeV/c2. This result is consistent with the most recent Tevatron average of 173.18 ± 0.94 GeV/c2 and will significantly improve the final combination.

—edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Hyun Su Lee (Ewha Womans University, Seoul, Korea), Jian Tang (University of Chicago) and Fermilab Deputy Director Young-Kee Kim.